This invention relates to an apparatus for the post-treatment, particularly the cooling, of articles which have been subjected to an isostatic pressing process in which the articles are subjected to high temperatures and pressures in a pressure chamber while stacked in a loading can which is located in a heating furnace.
In the treatment of articles by the isostatic pressing method, the articles are subjected to high pressures of from about 500 to about 2000 bar and high temperatures of from about 500.degree. C. to about 2000.degree. C. while stacked on a loading can or loading rack. This loading can rests on a base in the center of a furnace which is contained in a closed pressure chamber. The furnace is generally formed of a network of electrical resistors which, in turn, is surrounded by a thermal insulating screen. A number of passages for the circulation of gas during the cooling phase are generally provided at the top of the insulating screen. A device equipped with a furnace of the type described generally consists primarily of a cylindrical pressure chamber with a vertical axis, the wall thickness of which is sufficient to withstand the high gas pressures developed in the apparatus.
In order to accelerate the cooling phase, it is known to provide a cooling valve at the bottom of the pressure chamber. A series of opening or pipes connect this cooling valve, and in particular the valve housing thereof, to a space around the insulating screen. These pipes discharge into the bottom of the pressure chamber which is filled with argon or another suitable inert gas.
When the valve is closed, cooling by natural convection is prevented. When the valve is open, cooling by natural convection takes place by circulation of the gas in the pressure chamber. The gas circulates from the bottom of the pressure chamber, at the level of the valve housing, towards the higher regions of the furnace. The gas flows out of the furnace proper through holes or openings provided at the top in the insulating wall of the furnace to the circular space between the furnace and the wall of the pressure chamber.
The circulation of the gas by natural convection is not, in itself, sufficient to promote the rapid cooling of the treated molded articles. This is because the use of the known structure produces a stratification phenomenon at the top of the pressure chamber with the result that the very hot gases stabilize themselves at the top of the pressure chamber and the less hot gases stabilize themselves at the bottom of the pressure chamber and in the furnace.
Further problems arise in this connection because the rate of cooling is dependent on the physical properties of the gas used, such as the density/temperature relationship, which may result in nonuniform cooling of the articles stacked in the furnace. The physical structure of the treated product may not be the same for all of the articles because of different or diverging cooling gradients.
The object of the present invention is to overcome these and similar drawbacks of known apparatus and to provide an apparatus in which a considerably faster cooling of articles, under very similar circumstances, can be expected.